Pigura field is one of PT. Chevron Pacific Indonesia field located in Rokan PSC block, Riau province just above oil prolific Central Sumatra Basin (CSB). Major reservoir of this field is Melawai sand, which has average effective porosity (PHIE) 27% with Darcy permeability. More than 50% of total field OOIP and major oil production since 1981 to date is recover from this sand. Therefore, it is very important to have a valid OOIP number of Melawai sand for future strategic field development. OOIP is calculated using deterministic method and probabilistic method which both methods constructively give big picture about our OOIP. By applying both methods, we could have single number together with range of OOIP and understand about uncertainties that have direct impact to OOIP calculation result. Deterministic calculation method is relatively common to calculate OOIP but it tends to exclude uncertainty of reservoir properties such as net to gross (NTG), effective porosity (Ø), irreducible water saturation (SWirr), because we only use single average number of each properties. Deterministic uses Average Value (1D), ZMAP (2D) and Gocad (3D) approaches with result are 63 MMBO, 53 MMBO and 43 MMBO respectively. Fortunately, uncertainties could cover using probabilistic method, where we honor reservoir properties data distribution into OOIP calculation. In this method, we are using Monte Carlo Simulation as an integral part of Crystal Ball software. This calculation method resulting probability distribution of Melawai OOIP that can be simplified become 3 (three) categories those are 15.96 MMBO for P10 (Low), 39.98 MMBO for P50 (Medium), and 76.07 MMBO for P90 (High). At the end of our work, we have range of OOIP expand from P10 to P90 and deterministic result located between P50 to P90 probabilistic OOIP range. Based on our level of confidence and cumulative oil production data, we could decide which OOIP value that applicable to our field and also aware reservoir properties, in this case are gross thickness and NTG, which have highest impact to our OOIP calculation.